In recent years, saving in energy resources in various fields has received attention and has caused a ripple effect also on the field of power supply, for example. Specifically, higher efficiency in a switching power supply, for example, has been sought.
The switching power supply converts an input direct voltage into a desired direct voltage by using a DC to DC (Direct Current to Direct Current) converter and thereby outputs the resultant voltage as a regulated power supply voltage. With a decrease in voltage and an increase in power consumption in a semiconductor element such as a CPU (central processing unit) for use in a computer, a switching power supply capable of achieving both low voltage and large-current output has come into use also as a power supply unit.
For further improvement in output efficiency of the power supply unit, power consumed by a switching element (i.e. a switching transistor) for use, for example, may be reduced. For a reduction in power loss, it is therefore desirable to use a switching element having low on-state resistance and a high switching rate. There has been developed a switching power supply transistor using a compound semiconductor material rather than silicon, which satisfies both these resistance and rate characteristics.
The followings are reference documents.    [Document 1] Japanese Laid-open Patent Publication No. 53-048426,    [Document 2] Japanese Laid-open Patent Publication No. 2007-159364, and    [Document 3] Japanese Laid-open Patent Publication No. 2008-182884.